Mingxing Wang

Impacts of soil moisture on nitrous oxide emission from croplands: a case study on the rice-based agro-ecosystem in Southeast China

Abstract Based on the in situ measurement of soil moisture and nitrous oxide (N2O) emission from a rice–wheat rotation ecosystem of southeast China and on the simulated experiments in laboratory, the impact of soil moisture on N2O emission is investigated. By analyzing the experimental data in detail, some results could be outlined as follows: (a) It is soil moisture and temperature instead of N fertilization that determines the seasonal variation pattern of N2O emission from the rice-based crop rotation ecosystem of southeast China. (b) Soil moisture is the most sensitive factor to regulate N2O emission from croplands. (c) Explosive emission of N2O from the rice-based agro-ecosystem was found to happen at the soil moisture within (110±5)% soil water holding capacity or field capacity (SWHC) or (99±9)% water-filled pore space (WFPS). When soil moisture of the rice–wheat fields is less than 105% SWHC, the N2O emission was observed to increase exponentially vs. soil moisture. In contrast, N2O emission was found to decrease reciprocally vs. soil moisture more than 115% SWHC. (d) The response of the N2O emission rate from soils in fields to variations of soil moisture may be well described with a general empirical equation. For x⩽C0% SWHC, F=A e −B(x−C 0 ) 2 +D e Ex . For x⩾C0% SWHC, F=A e −B(x−C 0 ) 2 + e G x −H . The equation to describe the relationship between soil moisture and N2O emission rates from incubated soil is different from that for fitting data observed in fields. Reasons for the difference still remains uncertain.

A comparison between measured and modeled N2O emissions from Inner Mongolian semi-arid grassland

The objectives of this study were (1) to determine the effect of land use on N2O emissions from Inner Mongolian semi-arid grasslands of China and (2) to evaluate the process-based DNDC model to extrapolate our field measurements from a limited number of sites to a larger temporal and spatial scale. The results suggest the following. Rainfall event was the dominant controlling factor for the seasonal variations of the N2O fluxes. The seven selected sites exhibited a similar seasonal trend in N2O emission, despite their different vegetation, land use and textures. In the typical steppe, N2O fluxes generally decrease with decreasing soil organic C (SOC) and total N content, indicating that soil C and N pools are very important in determining the spatial magnitude of the N2O flux. N2O emissions were very small during the entire growing season, averaging only 0.76 g N2O-N ha−1 day−1 for the five typical steppe sites, 0.35 g N2O-N ha−1 day−1 for the mown meadow steppe site, and 0.83 g N2O-N ha−1 day−1 from the cropped meadow steppe site. No enhanced effect due to overgrazing was observed for the N2O emission from the semi-arid grasslands. This was mainly results from the decreased SOC content due to overgrazing, which may have reduced the promoting effect of increased soil bulk density by trampling and animal excreta. Except for the mown steppe site, the model predictions of the N2O flux for the six different sites agree well with the observed values (r2 ranging from 0.35 to 0.68). It would be concluded that the DNDC model captured the key driving process for N2O emission. Nitrification was the predominant process, contributing 64–88% to the N2O emission. However, in terms of the magnitude of the N2O emission, further modifications should focus on the underestimated N2O flux during the spring and autumn periods (nitrification, freeze/thaw cycles) and the effect of topography and the mowing on N2O emission.

A one-compartment model to study soil carbon decomposition rate at equilibrium situation

Abstract Based on world-wide observed soil carbon density data and model-simulated net primary production (NPP), we obtained a one-compartment model directly calculating average soil carbon respiration rate and soil carbon density at equilibrium situation. Our formulation shows that the average decomposition rate of soil organic matter, both at dry and wet soil condition, is particularly sensitive to moisture/precipitation changes and it is also sensitive to temperature. The data show that, at wet soil condition, decomposition rate decreases with moisture increasing, while at dry soil condition, it increases with moisture increasing. These results are consistent with previous studies. The temperature effect on soil respiration rate is evident, with Q10=1.7. The model has a good resolution to describe soil carbon densities at various vegetation types in the Holdridge life ecosystem classification system. Results show that the highest soil carbon densities occur in rain forest/tundra, while the lowest values in deserts and the highest soil respiration rates occur in wet forest/tundra but not in rain forest/tundra, while the lowest rates occur in desert. The calculated present terrestrial carbon pool is ≈1200 PgC. On the basis of model results, some recent conclusions indicating that soil organic carbon decomposition does not vary with temperature should be revised. The soil respiration rates are highly dependent on the combined effects of temperature and precipitation, but not any individual effect. High precipitation rate or soil moisture will greatly prevent soil organic matter from decomposition.

Methane emission from a simulated rice field ecosystem as influenced by hydroquinone and dicyandiamide.

A simple apparatus for collecting methane emission from a simulated rice field ecosystem was formed. With no wheat straw powder amended all treatments with inhibitor(s) had so much lower methane emission during rice growth than the treatment with urea alone (control), which was contrary to methane emission from the cut rice-soil system. Especially for treatments with dicyandiamide (DCD) and with DCD plus hydroquinone (HQ), the total amount of methane emission from the soil system and intact rice-soil system was 68.25-46.64% and 46.89-41.78% of the control, respectively. Hence, DCD, especially in combination with HQ, not only increased methane oxidation in the floodwater-soil interface following application of urea, but also significantly enhanced methane oxidation in rice root rhizosphere, particularly from its tillering to booting stage. Wheat straw powder incorporated into flooded surface layer soil significantly weakened the above-mentioned simulating effects. Regression analysis indicated that methane emission from the rice field ecosystem was related to the turnover of ammonium-N in flooded surface layer soil. Diminishing methane emissions from the rice field ecosystem was significantly beneficial to the growth of rice.

Trends of atmospheric methane in Beijing

Abstract The concentration of atmospheric methane in Beijing is still increasing, although its annual trend has significantly decreased from 2.0% yr −1 in 1985–1989 to 0.5% yr −1 in 1990–1997. The seasonal variability of methane concentration apparently appeared in a double-peak pattern with one peak in winter and the other in summer. It is known that the seasonal inter-annual variations of atmospheric methane in Beijing are different from year to year. From 1986 to 1997, the concentrations of atmospheric methane increased by 184 ppbv, in which about 37% was due to its increase in winter and 21% in summer. After 1993, the trends of methane concentration in summer, which are mainly due to emission from biogenic sources, are negative, while their trends in winter, which are mainly due to emission from non-biogenic sources, are positive with a value of about 25 ppbv yr −1 . As a result, the seasonal inter-annual trends from 1993 to 1997 were mainly due to the increase of methane emission from non-biogenic sources in winter. It implies, therefore, that in Beijing the biogenic sources have been decreasing but the non-biogenic ones, such as fossil fuel combustion, have increased.

Long-term trends of carbon monoxide inferred using a two-dimensional model

Abstract A global two-dimensional chemistry model is used to study the long-term trends of CH4, CO, and OH from pre-industrial times to 2020 with given emission scenarios according to the increase of world population. The calculated global-averaged concentration of CO is 27 ppbv before 1840, the concentration of CO is 76 ppbv in 1991, and is estimated to be 105 ppbv in 2020. From 1840 to 1991, the concentration of OH changed from 7.17×105 to 5.79×105 molecules/cm3, i.e., decreased by 19%. The long-term trends of CH4 derived from the model are in good agreement with observation results. The annual increase of CH4 during 1983–1991 is 12.1–13.3 ppbv by this model and 11.1–11.6 ppbv by observation. The calculated growth rate of CO in 1980s is 1.03–1.06%/yr i.e., 6.9–7.9 ppbv/yr. The model is used to investigate why the CO concentration decreased at the beginning of 1990s. We find that the decrease of CO emissions and depletion of stratospheric ozone are the best explanation, which account for 70% and 30% of the decrease of CO concentration, respectively. The model results also show that possible reduction of CH4 emission has little influence on the change of CO concentrations though the reduction of CO emission can counteract the growth of CH4 significantly.

Effect of long-term organic fertilization on the soil pore characteristics of greenhouse vegetable fields converted from rice-wheat rotation fields

The shift from rice-wheat rotation (RWR) to greenhouse vegetable soils has been widely practiced in China. Several studies have discussed the changes in soil properties with land-use changes, but few studies have sought to address the differences in soil pore properties, especially for fields based on long-term organic fertilization under greenhouse vegetable system from RWR fields. This study uses the X-ray computed tomography (CT) scanning and statistical analysis to compare the long-term effects of the conversion of organic greenhouse vegetable fields (over one year, nine years, and fourteen years) from RWR fields on the soil macropore structure as well as the influencing factors from samples obtained in Nanjing, Jiangsu, China, using the surface soil layer and triplicate samples. The results demonstrated that the macropore structure became more complex and stable, with a higher connectivity, fractal dimension (FD) and a lower degree of anisotropy (DA), as the greenhouse vegetable planting time increased. The total topsoil macroporosity increased considerably, but the rate of increase gradually decelerated with time. The transmission pores (round pores ranging from 50 to 500μm) increased with time, but the biopores (>2000μm) clearly decreased after nine years of use as greenhouse vegetable fields. Soil organic matter (OM) has a significant relationship with the soil pore structure characteristics, especially for the transmission pores. In addition, organic fertilization on the topsoil had a short-term effect on the pores, but the effect stabilized and had a weak influence on the pores over longer periods. These results suggested that organic fertilization was conducive for controlling soil degradation regarding it physical quality for water and oxygen availability in the short term.